Liquid-filled electric radiators are coming more and more into use. The liquid in such radiators is usually transformer oil, which has suitable properties for the purpose. The immersion heater used for heating the oil is arranged within the space in the radiator which is used for the oil flow, usually a collection duct situated downwards in the radiator. In such a case the heater is inserted in the collection duct from one short end of the radiator. The insertion opening then being closed by a liquid-sealing stub through which the electric supply lines to the heater pass out to the inside of a casing for electric components. The casing is thus arranged as an extension of the lower end portion of the radiator. Such a known radiator is described in French Pat. No. 1 586 411.
Such known liquid-filled electric radiators have a disadvantage in that the liquid is heated by the immersion heater at the lowest point of the radiator to obtain circulation by convection. The heat-emitting plates of the radiator will thus be warmer downwards than they are upwards, which has an unfavourable effect on the air intended to be heated by the radiator, since the air in the room where the radiator is placed normally circulates from the floor and up to the ceiling. The known liquid-filled electric radiators, which emit more heat downwards near the floor than at their upper portion situated closer to the ceiling thus have poor efficiency. The cold air stream in the room first meets the highest temperature of the radiator and takes up heat, while the heat exchange further up along the radiator surface will be poor, since the air has already become well-heated at its previous contact with the lower portion of the radiator.
A further disadvantage in the known liquid-filled electric radiators is that due to the greater heat in the downward portions, where the casing for the electric components is also situated, the casing has a high interior temperature. Measurements have shown that in a liquid-filled electric radiator with a surface temperature of 80.degree. C., the temperature inside the casing is approximately 40.degree. C. This makes the casing unsuitable, if not impossible, as a location for mounting the radiator thermostat.
To solve the disadvantages mentioned above it has been proposed to arrange the electric immersion heating element in a separate vessel. Liquid-filled radiators of this type are known from, e.g. French Pat. No. 922 393. The radiators are of a casted grill type having the separate vessel arranged under the radiator body. The vessel is connected to the end of the collection duct via a longer return tube and to the end of the distribution duct via a separate ascending tube for the liquid heated in the vessel. An inconvenient drawback of the radiator shown in the French patent is the demand for a thermostat controlling the energy supply from the electric heating element independent of the temperature of the return liquid. Another drawback is that the vessel, as well, as the tubes, are arranged outside the border lines of the radiator body.
Also, electric radiators of panel type are previously known, e.g. from Norwegian Pat. Nos. 61 522 and 59 936, where a vessel body having a circular cross section is arranged in a vertical position between two parallel-plane panel radiator bodies. The vertical vessel is connected to the collection and distribution ducts through transverse branch pipes and contains a concentric tube housing an electric heating element. Thus, there is only an indirect liquid heating which gives the radiator a poor efficiency. Contributing to the poor efficiency is also that no optimum self circulation of the liquid is obtained by a vertical heating element.